This invention relates to a combustion controlling apparatus having an igniter circuit and a flame-rod type flame detecting circuit, and particularly to one which is powered by a low voltage AC source and used for a pulse combustion apparatus.
The low voltage AC source is for example, a 24-volt source.
The pulse combustion apparatus is different from the ordinary combustion apparatus in which a flame of combustion is continuously present as time elapses, in that it generates flames intermittently, or in a pulse like manner. One of the various types of known pulse combustion apparatus, for example, has a combustion chamber, a valve for controlling gaseous fuel and air to be supplied to the combustion chamber, and a discharging tube so designed as to cause a resonant oscillation in the tube of a predetermined period in cooperation with the combustion chamber. In general, the combustion chamber and the discharging tube constitute a heat exchanger. In operation, a fuel-air mixture is supplied through the valve to the combustion chamber, where it is ignited by an igniter to be burnt explosively. Under the pressure produced by the explosive combustion, the valve is closed, while the burnt gas is discharged through the discharging tube. Due to the discharge of the exhaust gas a negative pressure prevails in the combustion chamber, whereby the valve is opened to again such in the gaseous fuel and air on one hand. On the other hand, the resonant oscillation in the tube produced through the cooperation of the combustion chamber and discharging tube causes the remaining combustion flame or high temperature gas in the discharging tube to return to the combustion chamber, so that the sucked air-fuel mixture is explosively burnt by the remaining combustion flame or high temperature gas. This increased pressure causes the previous exhaust gas to discharge. This operation as one cycle of combustion is repeated in succession. That is, the combustion is performed intermittently, or in a pulse-like manner.
The frequency of this pulse combustion is generally 50 Hz to 80 Hz.
In the flame-rod type flame detecting circuit, an AC voltage of about 100 to 150 V is applied between a pair of electrodes which are provided to contact flames, so that the variations in the applied AC voltage caused across the electrodes is detected by the rectifying action of flame. Thus, the variations in the AC voltage can be taken out as a signal representing the presence or absence of flame. Detection of flame is effected during a positive half-wave period of the AC voltage.
Thus, when the commercial AC voltage of 60 Hz is applied to the flame combustion at 50 Hz to 80 Hz, the positive half-wave voltage is not always synchronized with the presence of flame because of no correlation therebetween, and therefore the flame can not be detected. Accordingly, the frequency of the AC to be applied is increased to, for example, about 800 Hz so that the positive halves of the AC are synchronized with the pulse flames.
The conventional combustion control apparatus of the kind mentioned above will be described with reference to FIGS. 1 and 2.
In FIG. 1, an AC voltage from a low-voltage AC power source 1 is converted to a DC voltage by a rectifying circuit 2, and then converted and boosted to a high-frequency AC voltage by an inverter 3 which produces an output of a sinusoidal waveform. The voltage produced from the inverter 3 is applied to an igniting circuit 4 and a flame-rod type flame detecting circuit 5. An igniting electrode 6 ignites a burner 7 and a flame rod 8 detects flames. A control circuit 9 and a fuel valve driving circuit 10 are supplied with power from the rectifying circuit 2, and the control circuit 9 controls the operations of the igniting circuit 4, the fuel valve driving circuit 10 and a fuel valve 11.
The arrangement of FIG. 1 will be described in detail with reference to FIG. 2.
The AC power source 1 is turned on by a thermostat which is actuated depending on the temperature of the load. The rectifying citrcuit 2 is composed of a diode 20 and a capacitor 21. The inverter 3 has an oscillation circuit and a transformer 22, and the oscillator circuit is formed of a capacitor 23, resistors 24 and 25, a transistor 26 and capacitors 27 and 28. The output of the inverter 3 is a sine wave voltage with an amplitude of 100 to 150 V and a frequency of 800 Hz. When a small amount of collector current flows in the transistor 26 because of the fluctuation of source voltage or the like, the transistor 26 is turned on. When the charge on the capacitor 28 is discharged through the transformer 22, the transistor 26 is turned off. Then, when the transistor 26 becomes completely in the off state, it goes to the on-state depending on the time constant circuit of the resistor 25 and capacitor 27. The above operation is repeated.
The igniting circuit 4 includes a pulse generating circuit composed of diodes 30, 31 and 32, resistors 33, 34, 35 and 36, a thyristor 37, a diac 38, capacitors 39 and 40, and a transistor 41. The igniting circuit 4 further includes a pulse transformer 42. When the igniting circuit 4 is operated, the transistor 41 is turned off. The capacitor 39 for charge and discharge is charged through the diode 30. The capacitor 40 is charged through the resistor 34, and when the voltage across the capacitor 40 reaches a value which is large enough to make the diac 38 conductive, the charge of the capacitor 40 flows into the thyristor 37 as a gate current through the diac 38 and the resistor 35 to thereby turn the thyristor 37 on. Then, the charge on the capacitor 39 is discharged through the thyristor 37, the pulse transformer 42 and the diode 31, and therefore a high voltage for ignition is induced. The resistor 33 controls the charged voltage across the capacitor 39 when the igniting circuit 4 is not operating.
The flame detecting circuit 5 comprises capacitors 45 and 46, resistors 48 and 49 and a part of a combustion controlling integrated circuit 50. The AC source voltage for flame detection is applied through the capacitor 45 and rectified by the rectifying action of the flame into a DC current, the AC component of which is removed by the resistors 48 and 49 and the capacitor 46. The resulting DC voltage is applied to a flame signal input terminal 51 of the combustion controlling integrated circuit 50 of the control circuit 9.
The integrated circuit 50 may be, for example, the HA-16605 W made by Hitachi, Ltd., and performs sequence control. When the ignition starts, the integrated circuit 50 causes its output terminals 54 and 55 to be low level to permit a current to be supplied to an electromagnetic coil 56 of the fuel valve, thus fuel being supplied, on one hand, and to cause the igniter circuit 4 to operate on the other hand. When the burner is ignited the output terminal 55 becomes low in its level, so that the transistor 41 is turned on to stop the operation of the igniting circuit 4.
A resistor 58, a capacitor 59 and a zener diode 60 form a power soruce for the integrated circuit 50. Numeral 61 represents a power source input terminal, and 62 a ground terminal.
In this arrangement, since the inverter 3 feeds the secondary output of the transformer 22 back to the primary, the secondary voltage and frequency are changed depending on the value of the secondary load. Therefore, when the igniting circuit 4 is operated, a low voltage is applied to the flame rod 8, and a small flame current is obtained by the application of AC to flame, so that it is difficult to detect the flame during ignition.
Moreover, since a current is caused to always flow into the base of the transistor 26 of the inverter 3, the base current enters the active region of the transistor 26 at a certain time and thus the transistor 26 generates heat due to the collector loss.
Furthermore, since power is supplied from the rectifying circuit 2 to the control circuit 9 and fuel valve driving circuit 10 even if the inverter 3 breaks down, the control sequence progresses and the fuel valve opens despite disabled ignition, resulting in poor safety.